Spatial Light Modulators (SLMs) have found numerous applications in the areas of optical information processing, projection displays, video and graphics monitors, televisions, and electrophotographic printing. SLMs are devices that modulate incident light in a spatial pattern to form a light image corresponding to an electrical or optical input. The incident light may be modulated in its phase, intensity, polarization, or direction. The light modulation may be achieved by a variety of materials exhibiting various electro-optic or magneto-optic effects, and by materials that modulate light by surface deformation.
An SLM is typically comprised of an area or linear array of addressable picture elements (pixels). Source pixel data is first formatted by an associated control circuit, usually external to the SLM, and then loaded into the pixel array one frame at a time. This pixel data may be written to the pixel array using a variety of algorithms, i.e. sequentially top-to-bottom one pixel line at a time, interleaving by sequentially addressing top-to-bottom ever other pixel line, such as the odd rows of pixels, and then returning to address the even pixel lines, etc. In cathode ray tubes (CRTs), this data writing technique is know as rasterizing, whereby a high powered electron gun scans across the pixel elements of a phosphor screen left to fight, one line at a time. This pixel address data writing scheme is equally applicable to liquid crystal displays (LCDs) as well.
A recent innovation of Texas Instruments Incorporated of Dallas Tex., is the digital micromirror device or the deformable mirror device (collectively DMD). The DMD is an electro/mechanical/optical SLM suitable for use in displays, projectors and hard copy printers. The DMD is a monolithic single-chip integrated circuit SLM, comprised of a high density array of 16 micron square movable micromirrors on 17 micron centers. These mirrors are fabricated over address circuitry including an array of SRAM cells and address electrodes. Each mirror forms one pixel of the DMD array and is bistable, that is to say, stable in one of two positions, wherein a source of light directed upon the mirror array will be reflected in one of two directions. In one stable "on" mirror position, incident light to that mirror will be reflected to a projector lens and focused on a display screen or a photosensitive element of a printer. In the other "off" mirror position, light directed on the mirror will be deflected to a light absorber. Each mirror of the array is individually controlled to either direct incident light into the projector lens, or to the light absorber. The projector lens ultimately focuses and magnifies the modulated light from the pixel mirrors onto a display screen and produce an image in the case of a display. If each pixel mirror of the DMD array is in the "on" position, the displayed image will be an array of bright pixels.
For a more detailed discussion of the DMD device and uses, cross reference is made to U.S. Pat. No. 5,061,049 to Hornbeck, entitled "Spatial Light Modulator and Method"; U.S. Pat. No. 5,079,544 to DeMond, et al, entitled "Standard Independent Digitized Video System"; and U.S. Pat. No. 5,105,369 to Nelson, entitled "Printing System Exposure Module Alignment Method and Apparatus of Manufacture", each patent being assigned to the same assignee of the present invention and the teachings of each are incorporated herein by reference. Gray scale of the pixels forming the image is achieved by pulse-width modulation techniques of the mirrors, such as that described in U.S. Pat. No. 5,278,652, entitled "DMD Architecture and Timing for Use in a Pulse-Width Modulated Display System", assigned to the same assignee of the present invention, and the teachings of which are incorporated herein by reference.
The DMD is revolutionary in that it is truly a digital display device and an integrated circuit solution. The evolution and variations of the DMD can be appreciated through a reading of several commonly assigned patents. The "first generation" of DMD spatial light modulators implemented a deflectable beam wherein the mirror and the beam were one in the same. That is, an electrostatic force was created between the mirror and the underlying address electrode to induce deflection thereof. The deflection of these mirrors can be variable and operate in the analog mode, and may comprise a leaf-spring or cantilevered beam, as disclosed in commonly assigned U.S. Pat. No. 4,662,746 to Hornbeck, entitled "Spatial Light Modulator and Method", U.S. Pat. No. 4,710,732 to Hornbeck, entitled "Spatial Light Modulator and Method", U.S. Pat. No. 4,956,619 to Hornbeck, entitled "Spatial Light Modulator", and U.S. Pat. No. 5,172,262 to Hornbeck, entitled "Spatial Light Modulator and Method", the teachings of each incorporated herein by reference.
This first generation DMD can also be embodied as a digital or bistable device. The beam (mirror) can include a mirror supported by a torsion hinge and axially rotated one of two directions 10 degrees, until the mirror tip lands upon a landing pad. Such an embodiment is disclosed in commonly assigned U.S. Pat. No. 5,061,049 to Hornbeck entitled "Spatial Light Modulator and Method". To limit the Van der Waals forces between the mirror tips and the landing pads, the landing pads may be passivated by an oriented monolayer formed upon the landing pad. This monolayer decreases the Van der Waals forces and prevents sticking of the mirror to the electrode. This technique is disclosed in commonly assigned U.S. Pat. No. 5,331,454 to Hornbeck, entitled "Low Reset Voltage Process for DMD", the teachings included herein by reference.
A "second generation" of the DMD is embodied in commonly assigned U.S. Pat. No. 5,083,857 entitled "Multi-Level Deformable Mirror Device", as well as in copending patent application Ser. No. 08/171,303 entitled "Improved Multi-Level Digital Micromirror Device, filed Dec. 21, 1993. In this second generation device, the mirror is elevated above a yoke, this yoke being suspended over the addressing circuitry by a pair of torsion hinges. As depicted in FIG. 3c of this application, an electrostatic force is generated between the elevated mirror and an elevated electrode. When rotated, it is the yoke that comes into contact with a landing electrode, whereby the mirror tips never come into contact with any structure. The shorter moment arm of the yoke, being about 50% of the mirror, allows energy to be more efficiently coupled into the mirror by reset pulses due to the fact that the mirror tip is free to move. Applying resonant reset pulses to the mirror to help free the pivoting structure from the landing electrode is disclosed in commonly assigned U.S. Pat. No. 5,096,279, entitled "Spatial Light Modulator and Method, and U.S. Pat. No. 5,233,456 entitled "Resonant Mirror and Method of Manufacture". However, some of the address torque generated between the mirror and the elevated address electrode is sacrificed compared to the first generation devices because the yoke slightly diminishes the surface area of the address electrode.
It is desired to provide an improved DMD having a more efficient reset action, and to develop a device with more address torque, latching torque, and address holding torque. The improved device would preferably be fabricated using the baseline fabrication processes.